Grishka:

"I wrote about my impression (no proof, but good arguments) that British diplomacy could have prevented the war in the 190x years, to the immense benefit of their own country and of the world."

As the grounding for the causes of the First World War lay firmly in disputes centred on the relations between the Austro-Hungarian Empire and Serbia I doubt very much if there was anything that Great Britain could have done to prevent anything that subsequently happened. The only people who could have done that were those involved in the initial exchanges. Had the Austro-Hungarians accepted the Serbs offer for negotiations on the 30th July, as the German diplomats strenuously urged them to, then there would have been no Russian mobilisation on the 31st July and there would have been no German mobilisation on the 1st August - there would have been no First World War - (By the way the really ridiculous thing about it was that although the Austrians declared war on the Serbs on the 28th July 1914, the Austrians could not make any move to actually attack Serbia before the 12th August - they had all that time to defuse the situation - unfortunately neither the Russians, the Serbs nor the Germans actually knew that at the time.)

Pray tell in what 190x years prior to the assassination of the Archduke Franz Ferdinand were Great Britain, France, Russia, Austro-Hungary and Italy close to war? Where and when could this British diplomatic effort have been brought to bear?

All a nonsense really, all Great Britain could do throughout those years is do as she did, reassure allies, and warn potential foes of the consequences of certain actions on their part.